The present invention relates to a device in a burner gas turbines as appears in the preamble of claim 1.
Low emission gas turbine combustors are previously known from e.g. U.S. Pat. No. 5,816,050 and WO 9207221. The drives for low emission combustors are often counteracted by the additional cost and complexity of the injection system, the control system and the design of the combustor itself.
Not only oxides of nitrogen (NOx) must be considered but also emissions of carbon monoxide (CO), unburned hydrocarbons (UHC) and in the most severe cases also soot and other trace species. Furthermore the regulations of emissions from gas turbines are also moving into limiting the emissions in a wider operating range which poses serious problems for stability of the combustor, acoustic resonance and furthermore complexity. This is due to the nature of the most common emission control technique, lean premixed combustion (LP), which offers less stability than the traditional high emission diffusion flame combustion (DF).
In European Patent Application 656 512 (Westinghouse Electric Corporation) a device in a burner for gas turbines is described, comprising a housing with a centrally located a fuel inlet tube, surrounded by two concentric annular chambers extending into an extended diameter combustion chamber. The fuel inlet from a source of fuel is provided through the centrally located tube. Means for supplying combustion air to said annular chambers being provided with radial flow swirlers for creating contra rotating movements of the combustion air in said two annular chambers. The fuel inlet of this device is aiming directly into a primary burning zone created in a vortex at the free and of the fuel inlet tube.
This arrangement creates a short burning zone, with a very high temperature at the end of the fuel inlet tube. The short burning zone will create undesirable emissions. Further, the arrangement of two concentric annular chambers makes severe restrictions in the minimum size of this burner, together with the mere complexity of the arrangement with a multitude of injection points, swirler vanes and passages.
There is a main object of the present invention to provide for an improved burner for gas turbines.
It is a further object to provide a burner for gas turbines, which can be designed for a wide range of capacities and which can utilised within a wide range of operating conditions.
It is a still further object, to provide a simple and cost effective technology for emissions reductions.
The device according to the present invention is defined in the characterizing clause of the following independent claim 1. Preferred embodiments of the device appears from the dependent claims.
As described in the introduction, the object of the present invention is to enable low emissions of NOx and CO over a wide operating range in a low complexity, cost effective configuration.
The burner can operate as a single stage burner, or as a multi stage burner with different orientation of the secondary stage, either as a tangentially positioned venturi combustion zone or as a co-axial secondary stage of similar design.
In a first embodiment of a burner according to the invention, the air is fed through a plurality of radial extending feed channels where swirl is imposed to the air. This creates a swirling flow in the swirler cup annulus where fuel, liquid and/or gas, is fed via nozzles in the main central hub, also comprising a centrally locating spark plug for ignition at start-up. For load variations, fuel can also be supplied in the swirler. The air is swirled in the burner cup and is then forced through an converging conical outlet of the swirl cup. This configuration creates a strong swirling flow at the entrance to the main combustion zone. At the onset of combustion a vortex breakdown zone is formed with exhaust gas recirculation constituting a stable ignition source and helps in reducing emissions by lowering the reaction temperature. The gradual admixing of fuel and air through the main central gas supply acts as a aerodynamic multi stage combustion zone lowering the emissions. Furthermore, perfect mixed fuel and air mixes into the central flame at higher power settings through the mixing in the swirl feed channels. The conical outlet also has the effect of stopping flame flash-back of the premix flow due to the velocity increase it causes.
In contrast to prior art technologies, this invention promotes mixing from the central fuel injector to improve stability, but on the other hand the gradual admixing of fuel and air and the exhaust gas recirculation caused by the vortex break down reduces the reaction temperature to a level where low emissions can be achieved. Furthermore this can be achieved without any moving parts or by means of heat exposed nozzle devices.
Two further embodiments of the burner is described, one where the combustion process is divided into two separate fuel and air inlet ports, in both cases as a pilot burner providing excellent stability of the combustor system.
In the second embodiment, the secondary (or main) fuel and air inlet port is comprised of a tangentially entering venturi (Laval nozzle) to the main combustion chamber, comprising of a cylindrical tube being open at the other extreme where the hot gases leave the combustor for doing work in the subsequent turbine stages. The venturi premixing fuel and air device is also described in U.S. Pat. No. 5,638,674 and in No. 303551, but the combination of the first embodiment burner with a venturi is not describes elsewhere. In contrast to the mentioned US-patent, no moving parts are embodied in the invention and the venturi acts as the main mixing device being supported by the basic burner. The typical shortfalls of venturi premixers of low stability and limited range is thus overcome by the first embodiment of the burner which provides hot exhaust for stable ignition and that by shifting the load from the pilot to the venturi, low emissions can be achieved over a wider range.
In the third embodiment, the secondary (or main) fuel and inlet ports consists of an annular passage being coaxial to the basic pilot burner, but consists of the same elements as the basic burner. The first embodiment of the burner now comprises the central fuel injection tube and a radially extending swirler at the inlet of the main burner. The flow of the secondary burner is co-swirling to the basic burner flow. In contrast to e.g. U.S. Pat. No. 5,816,050, the flows are co-swirling and the outlet of the pilot and main burners comprise of two converging cones, giving a considerable increase in stability and exhaust flow recirculation.